An experimental investigation of heat transfer through a three-dimensional boundary layer has been performed. An initially two-dimensional boundary layer was made three dimensional by a transverse pressure gradient caused by a wedge obstruction, which turned the boundary layer within the plane of the main flow. Two cases, with similar streamwise pressure gradients and different lateral gradients, were studied so that the effect of the lateral gradient on heat transfer could be deduced. The velocity flowfield agreed with previous hydrodynamic investigations of this flow. The outer parts of the mean velocity profiles were shown to agree with the Squire-Winter theorem for rapidly turned flows. Heat transfer data were collected using a constant heat flux surface with embedded thermocouples for measuring surface temperatures. Mean fluid temperatures were obtained using a thermocouple probe. The temperature profiles, when plotted in outer scalings, showed logarithmic behavior consistent with two-dimensional flows. An integral analysis of the boundary layer equations was used to obtain a vector formulation for the enthalpy thickness, H 
H0ρuisdyρii,o(u2+w2)1/2,0,0ρwisdyρis,o(u2+w2)1/2
(where is is the stagnation enthalpy), which is consistent with the scalar formulation used for two-dimensional flows. Using the vector formulation, the heat transfer data agreed with standard two-dimensional correlations of the Stanton number and enthalpy thickness Reynolds number. It was concluded that although the heat transfer coefficient decreased faster than its two-dimensional counterpart, it was similar to the two-dimensional case. The vector form of the enthalpy thickness captured the rotation of the mean thermal energy flux away from the free-stream direction. Boundary layer three dimensionality increased with the strength of the transverse pressure gradient and the heat transfer coefficients were smaller for the stronger transverse gradient.
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